However, doing so would probably be a waste of time. The content pipeline is intended to provide build-time transformation services to non-code assets to prepare and optimize them for use at runtime (dynamically adding content at runtime is possible, though a bit tedious, if I recall).

You're talking about executable code in your C# files, and since the content pipeline's transformation services are a build-time operation there will be no functional difference between abusing the content pipeline to hold your C# files and simply adding those same C# files to your main game project, as Drackir suggested in his comment.

In fact, even if you leave the files as unprocessed source code or write a custom content manipulation step to compile them somehow, you'd be faced with the fact that many of the APIs available to compile code, or execute compiled code, at runtime aren't available on the non-desktop platforms -- so if you intend your XNA project to run on those, you almost certainly can't use the content pipeline.